1. Field of the Invention
The present invention relates to the creation and use of generalizations of classical watermarks for object identification, and more specifically, it relates to a relatively covert, xe2x80x9cwatermarkxe2x80x9d expressed in gamma-ray-emitting materials affixed to objects and employed for object identification. The xe2x80x9cgamma watermarkxe2x80x9d of the present invention is a type of steganography, or xe2x80x9chidden writing,xe2x80x9d which employs tiny quantities of material containing radionuclides to encode and continuously express a digital bit-string which may, for instance, be used to connote ownership of, or some type of prior contact with, an object whose provenance is in some manner contested or doubted.
2. Description of Related Art
A need exists for greatly improved means for general-purpose object identification. For example, a need exists for apprehending those responsible for theft of rare human artifacts and paleontological specimens, as a rapidly growing to problem is posed by escalating fossil and artifact thefts worldwide. A need exists for a broadly applicable means of labeling all such objects with a physically essentially-invisible, zero-hazard and relatively inexpensive xe2x80x98tagxe2x80x99 which could be discerned and then xe2x80x98readxe2x80x99 unequivocally only by well-equipped and expert individuals (e.g., law enforcement officials). Familiar tagging means is such as bar-codes, while eminently readable, also are readily detectable and often may be easily removed or altered. Digital watermarking of collections-of-bits encoding audio or graphics information is applicable only to bit-strings whose low-order bits may be manipulated for encoding purposes without damage to the perceived content of the collection-of-bits, a quite scope-limited though increasingly important type of property.
It is an object of the present invention to provide a gamma watermark containing a unique digital signature comparable in salient qualities to that of the digital watermark, but which may be applied to identify essentially, all items implemented as greater than microscopic-sized material objects.
It is another object, albeit an optional one, to provide, within a gamma watermark, a built-in xe2x80x98clockxe2x80x99 providing a date-stamp representation of the time of gamma watermark creation relative to the time at which the gamma watermark is being read, i.e., an age of the watermark.
Another object of the invention is to provide a gamma watermark that is undetectable by ordinary technical-inspection means (e.g., use of UV-fluorescence-stimulating illumination, magnified visual inspection, acoustic scanning, chemical treatment of an object""s surface, x-ray inspection, etc.),
Still another object of the invention is to provide a gamma watermark having an ultra-low radiation xe2x80x98signaturexe2x80x99 hidden in the ubiquitous natural background radiation due to cosmic radiation and natural plus man-made radioactivity in the environment (e.g., that due to decay of potassium 40, a billion-year half-lived isotopic component of natural potassium).
An object of the invention is provide a gamma watermark having an effectively microscopic physical size in order to enable second-level covertness-of-tagging and to confer sweep-resistance and counterfeit-robustness by owner-determined selective positioning on or within an object.
The gamma watermark is a new type of very low-level (i.e., nanoCurie-scale) gamma-ray-emitting tag or xe2x80x9cwatermarkxe2x80x9d, comprised of a sufficiently precisely metered, typically unique mixture-ratio of very small (of the order of 1 nanoCurie, or 10xe2x88x929 Curie) quantities of radioisotopes of appropriately long half-lives, none of which occur naturally (at levels as high as 1 nanoCurie) in the object to be tagged. The tag""s location may be variable, ranging from surface emplacement to cm-scale depth inside a full-density object (composed, e.g., of plastic, wood, stone, etc.) because MeV-energy gamma-rays are quite penetrating. In creating the tag, the ratios of the quantities of radioisotopes selected to comprise any given tag are made to be sufficiently precise to encode a binary bit-string with adequate xe2x80x9cnoise marginxe2x80x9d for unequivocal read-out at all subsequent times-of-interest and, if required by the particular tagging application, to be sufficiently unique among the tags applied to the class of objects that will ever be identified with such watermarks. Selected radionuclides package a huge amount of energy per gram and release it at a known rate for decades, so that gamma watermarks may be created which are fully useful over multi-decade intervals.
Radionuclides chosen for constituting a gamma watermark are either not present in the environment or are present at very low levels, so that a gamma watermark signatures may be very xe2x80x98cleanxe2x80x99 in the signal-to-noise sense. It is also possible to use radionuclides which are quite difficult to prepare, for example one which have unique production signatures betraying their means of generation. A number of radionuclides (e.g., 44Ti) of interest from these perspectives may be produced only, by spallation or charged-particle bombardments. Others have unique isotopic purity by virtue of using mass-separated target material or mass-separation after production. Use of such nuclides in gamma watermarks can drastically raise the threshold of endeavor for a would-be watermark counterfeiter, due to their uniqueness or the difficulty of obtaining them.
The nature of the gamma watermark is particularly convenient in many applications because gamma rays are peculiarly penetrating electromagnetic radiations. Many normal structural materials (e.g., wood, common plastics) have density xcfx81xcx9c1 gm/cc, while fossilized bone can have xcfx81=2-3 gm/cc and paper typically has xcfx81=1 gm/cc. Since the mass absorption coefficient of light elements such as carbon, nitrogen, oxygen, magnesium, aluminum and silicon for photons of 1 MeV energy, hereinafter referred to as 1 MeV xcex3s, is xe2x89xa60.04 cm2/g, the transport mean free path for such MeV-energy gamma-rays in all such materials is 25 g/cm2, e.g., 25. cm in 1 gm/cc material. Therefore, a gamma watermark could be implanted at a one-inch depth in low-to-moderate Z material and 90% of the emitted gamma-rays would still travel without scattering or absorption to a detector positioned over the material""s surface.
Similarly, a gamma watermark can be easily detected through modest stack-heights (a few cm) of paper. In fact, the activity level of the gamma watermark on any given paper-sheet could be made exceedingly small (picoCurie level), if working with stacks of paper all of which were so watermarked in a (nearly) identical manner; a detector used to examine a sheaf of such individually watermarked paper-sheets would xe2x80x9cseexe2x80x9d all the separate-but-identical watermarks superimposed into one which could be readily read out.
The salient components of standard physical theory underlying the gamma watermark include nuclear beta-decay and gamma-ray spectroscopy, semiconductor-based detection of ionizing radiation and viscous fluid-mechanical theory underlying ink-jet printers leveraged to enable high precision, swift creation of tokens in the direct gamma-watermarking of sheets of material such as plastic and paper. The gamma watermark (typically, redundantly) encodes its age (i.e., the time-elapsed since its creation) and a unique digital signature in the sufficiently-precisely-metered relative quantities of several different species of long lived, gamma ray-emitting radioisotopes. Because the photonic output of the beta-decay of a single atomic nucleus may be recorded with high efficiency and high precision, the amount of beta radioactivity needed to continuously express a unique digital signature may be made to be exceedingly small, at most 1 nanoCurie in many applications.
From a communications engineering perspective, the gamma watermark utilizes very low effective radiated power and very high spectral brightness at certain very narrowly defined energies/frequencies to xe2x80x9cnarrow-castxe2x80x9d a low-probability-of-intercept signal, using a long-lived, high-reliability nuclear power supply.
In the watermark""s built-in clock, at least two radioisotopes are employed to encode the date of creation of the tag at which time the ratio of the intensities of trio gamma-ray-emitting transitions of the trio radioisotopes of different half-lives is made to be equal (to a sufficiently precisely extent for subsequent read-out purposes) in the watermark-tag. At any later time, the then-observed ratio of the line-intensities, which may be determined as precisely as desired by increasing the counting interval, from these two (or more) transitions of known half-life constitutes a xe2x80x98clockxe2x80x99 from which xe2x80x98elapsed timexe2x80x99 can be traced back. Two or more such clocks can be so encoded (e.g., by employing radioisotopes of widely differing half-lives) by the use of three or more radioisotopes.
The content of the Watermark""s digital signature (a sequence of bits in a binary bit-string potentially dozens of bits in maximum length) is encoded in a manner basically similar to the clock(s). The ratio of the time-zeroed line intensity of the gamma radiation from a radioisotope to any reference line intensity encodes in an analog format of the magnitude of a string of binary digits, i.e., the binary fraction-expressed line-intensity ratio. Each radioisotope comprising a Watermark contributes a short string of bits (in most applications, 1-5 bits) to the total digital bit-string content of the Watermark. For purposes of reading-out this bit-string, the ratio of the line intensity of the gamma radiation from a radioisotope to a reference line intensity (e.g., that of the highest-energy line emitted by the longest-lived radioisotope used in the gamma watermark""s clock) is translated back to the time-of-creation of the Watermark using the time-interval encoded in the particular watermark""s clock.
The amount of information coded per radioisotope/radionuclide is user-selectable and depends on the amount of radioisotope used and its half-life relative to the specified effective lifetime of the watermark, the time interval available for readout of the Watermark""s contents, and the desired robustness of the readout (e.g., the degree of error syndrome-encoding, and thus redundancy, in the digital signature).
If faster readout or reduced total activity is desired for a Watermark containing any specified number of binary bits, then the total desired activity may be partitioned among more radioisotopes, each with a smaller amount of activity. This means that doubling the number of radioisotopes and cutting the total activity per radioisotope by 4-fold (e.g., the gamma watermark total activity drops by 2-fold) results in a required total Watermark readout time that is decreased by a factor of 4 (e.g., the first Watermark encodes 4 bits per radioisotope, while the second encodes only 2 bits on each of twice the number of radioisotopes, etc.)
To enhance the integrity of the gamma watermark, its digital signature may be error-syndrome, e.g., Hamming, -coded, e.g., in order to permit automated detection of 2-bit errors and detection-and-correction of single bit errors anywhere within its bit string. This feature makes feasible the objective expert certification of the digital content and integrity of the Watermark and also permits automated, minimum-elapsed time readout of the digital content of the Watermark.
If a binary bit-string of information is to be encoded N bits per radioisotope, i.e., as a binary-fraction specifying the intensity of a given spectral line emitted by a single radioisotope comprising a portion of the radiological inventory of a gamma watermark, then xcx9c(3xc3x972N)2 gamma-ray counts of that spectral line need to recorded, in order to have a statistically reliable estimate of the relative intensity which is statistically reliable at the level of three standard deviation about the true mean value. To get N binary bits of line-intensity information, the line-strength must be read out to 1 part in 2N. (I.e., five (5) bits require 1 part in 32, while four (4) bits require 1 part in 16, three (3) bits require 1 part in 8, two (2) bits require 1 part in 4, and one (1) bit requires 1 part in 2.) Thus, to generate a spectral peak amplitude of the required precision when reading-out a five-bit code, 3xc3x97(25=32)2 or 3072 counts are needed at that spectral-line energy, while for 2 bits, only 3xc3x9742=48 counts are needed to read out the spectral peak strength to the required precision. Of course, signals from many radioisotopes may be read out concurrently during the same xe2x80x98time intervalxe2x80x99 with a standard high-resolution gamma-ray spectrometer, as they each have at least one distinct-and-unique gamma-ray spectral energy.
A typical, in-the-field exemplary method for mass-producing gamma watermarks uses inkjet printers of the type often used with personal computers, representative unit costs of which at retail currently are xe2x89xa6$500. Various (e.g., 7) ink reservoirs in the ink cartridges of a single xe2x80x9cphotographic qualityxe2x80x9d color printer are loaded with radioisotopes in solution wherein one (1) radioisotope at precisely known concentration is loaded per reservoir. Computer software (e.g., the manufacturer""s inkjet printer-driver) may be used to write the xe2x80x9cdigital signaturexe2x80x9d constituting the Watermark by issuing appropriate low-level commands to the printer. For example, 29 drops of xe2x80x9cinkxe2x80x9d are dispatched from reservoir number 1, 17 drops from reservoir number 2, 4 drops from reservoir number 3, 21 drops from reservoir number 4 and 15 drops from reservoir number 5. This would serve to encode a bit string of 1110110001001001010101101, because the series of 5 binary bit strings equals 29, 17, 4, 21 and 15, i.e., 11101=29, 10001=17, 00100=4, 10101=21 and 01101=15. The Watermark""s clock is introduced by adding, e.g., 64 drops each from reservoirs #6 and #7. The above steps might be repeated until, for instance, 1,000 spatially separated watermarks have been so written, with each watermark thus having a distinct (typically, unique) computer program-controlled digital content. Potentially, all 1000 Watermarks could be placed on a single sheet of paper, e.g., each associated with a readily legible label. Mass-market inkjet printers currently write at 600 dots per inch, so a single Watermark would cover less than 10xe2x88x925 inches2, if the ink-dots of each Watermark were written on top of each other. The Watermarks so synthesized (and, if desired, labeled) could then be partitioned and packaged.
Among the objects which are amenable to inkjet application are stick-on labels and/or objects that are designed to be Gamma Watermarked (such as CD-ROMs). The position of the tag may be visually invisible or may be included as part of the specific text or symbols (e.g., a logo) by either under or overprinting or even including the radionuclides of the gamma-emitting tag in the visible ink itself. The inkjet printer version of the Gamma Watermark technology can also be used to directly xe2x80x98tagxe2x80x99 objects, e.g., the inkjet tagging capability may be used directly on objects to be watermarked, without resorting to prior printing of these tags on paper and later (e.g., paste-on) emplacement on the objects.